Commit cd77849a authored Jun 01, 2018 by Josh Poimboeuf Committed by Thomas Gleixner Jun 06, 2018

objtool: Fix GCC 8 cold subfunction detection for aliased functions

The kbuild test robot reported the following issue:

kernel/time/posix-stubs.o: warning: objtool: sys_ni_posix_timers.cold.1()+0x0: unreachable instruction

This file creates symbol aliases for the sys_ni_posix_timers() function.
So there are multiple ELF function symbols for the same function:

23: 0000000000000150 26 FUNC GLOBAL DEFAULT 1 __x64_sys_timer_create
24: 0000000000000150 26 FUNC GLOBAL DEFAULT 1 sys_ni_posix_timers
25: 0000000000000150 26 FUNC GLOBAL DEFAULT 1 __ia32_sys_timer_create
26: 0000000000000150 26 FUNC GLOBAL DEFAULT 1 __x64_sys_timer_gettime

Here's the corresponding cold subfunction:

11: 0000000000000000 45 FUNC LOCAL DEFAULT 6 sys_ni_posix_timers.cold.1

When analyzing overlapping functions, objtool only looks at the first
one in the symbol list. The rest of the functions are basically ignored
because they point to instructions which have already been analyzed.

So in this case it analyzes the __x64_sys_timer_create() function, but
then it fails to recognize that its cold subfunction is
sys_ni_posix_timers.cold.1(), because the names are different.

Make the subfunction detection a little smarter by associating each
subfunction with the first function which jumps to it, since that's the
one which will be analyzed.

Unfortunately we still have to leave the original subfunction detection
code in place, thanks to GCC switch tables. (See the comment for more
details.)

Fixes: 13810435 ("objtool: Support GCC 8's cold subfunctions")
Reported-by: kbuild test robot <lkp@intel.com>
Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: https://lkml.kernel.org/r/d3ba52662cbc8e3a64a3b64d44b4efc5674fd9ab.1527855808.git.jpoimboe@redhat.com

parent 716a685f

tools/objtool/check.c

+22 −0

Original line number	Diff line number	Diff line
		@@ -543,6 +543,28 @@ static int add_jump_destinations(struct objtool_file *file)
		dest_off);
		return -1;
		}

		/*
		* For GCC 8+, create parent/child links for any cold
		* subfunctions. This is _mostly_ redundant with a similar
		* initialization in read_symbols().
		*
		* If a function has aliases, we want the first such function
		* in the symbol table to be the subfunction's parent. In that
		* case we overwrite the initialization done in read_symbols().
		*
		* However this code can't completely replace the
		* read_symbols() code because this doesn't detect the case
		* where the parent function's only reference to a subfunction
		* is through a switch table.
		*/
		if (insn->func && insn->jump_dest->func &&
		insn->func != insn->jump_dest->func &&
		!strstr(insn->func->name, ".cold.") &&
		strstr(insn->jump_dest->func->name, ".cold.")) {
		insn->func->cfunc = insn->jump_dest->func;
		insn->jump_dest->func->pfunc = insn->func;
		}
		}

		return 0;

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